Unraveling a 150-Year-Old Enigma, <em>Psalidodon rivularis</em> (Acestrorhamphidae: Acestrorhampinae) a Species Complex or a Polymorphic Species?

Psalidodon rivularis, a fish endemic to the São Francisco River Basin and known as “piaba do córrego,” has long been regarded as a widely distributed species complex, exhibiting remarkable morphological and cytogenetic variation, even in sympatry. This study aims to determine whether P. rivularis represents a single polymorphic species or a group of cryptic species. We analyzed meristic, morphometric, and karyotypic data from 419 specimens identified as P. rivularis, as well as from the related species Astyanax turmalinensis and Hyphessobrycon santae. Additionally, we inferred the phylogeny of the group using NGS data from 25 individuals, incorporating both mitochondrial and nuclear genomic sequences. Our integrative results support the recognition of at least five distinct species within the P. rivularis complex. The true P. rivularis (called morphotype 1) has 46 chromosomes, while the others have 50 and differ in both morphology and distribution. One of these corresponds to Psalidodon santae comb. nov.—which includes A. turmalinensis as a junior synonym—and three others are newly described species. These findings clarify the diversity of fishes in the São Francisco River Basin and highlight the importance of conserving its unique freshwater ecosystems.